ACPI: Add ACPI namespace documentation

ACPI is implemented as a subsystem in Linux, it creates a device tree by
mapping specific ACPI namespace objects
(Device/Processor/PowerResource/ThermalZone) into Linux device objects.
This patch adds documentation for the ACPI device tree.

Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>